Ophthalmic tomography imaging apparatuses, such as an optical coherence tomography (OCT), can generate a three-dimensional image from retina layers. Such an image is generated by emitting a signal light beam to the retina, generating an interference light beam from a light beam reflected or scattered by each of the layers or boundary surfaces of the retina and a reference light beam, and forming an image using the interference light beam. By using this technology, the interior of the retina layers can be observed and, therefore, a disease can be correctly diagnosed. Accordingly, ophthalmic tomography imaging apparatuses have received a significant amount of attention in recent years.
It is important to correctly identify the layer structure of the retina in order to obtain an objective index for measuring the stage of disease progression (e.g., glaucoma progression). Examples of the index indicating glaucoma progression include a C/D ratio, which is a distance ratio between the edge of a depressed part of the optic papilla (referred to as a “cup”) and the edge of the optic papilla (referred to as a “disc”), the area, depth, and volume of the cup, and the thickness of the optic nerve fiber layer located between the inner limiting membrane and the external limiting membrane.
FIG. 14 is a schematic illustration of tomographic images of the optic papilla. As shown in FIG. 14, two-dimensional tomographic images (B-Scan images) T1 to Tn of the optic papilla are obtained by capturing the images of the retina in the depth direction of the retina. Each of the two-dimensional tomographic images includes a plurality of scan lines that scan the retina in the depth direction (hereinafter referred to as “A-scan lines”). A Z-axis represents the direction of the A-scan. By sequentially performing raster-scanning in a predetermined area of a plane on the retina (an x-y plane), the three-dimensional data including T1 to Tn can be obtained. Since the layers of the retina have different reflectivities, an inner limiting membrane 1401, a pigment layer of the retina 1402, and an optic papilla 1403, for example, can be identified by analyzing the image.
Citation List
Patent Literature
PTL 1: Japanese Patent Laid-Open No. 2008-246158
The shape of the optic papilla differs from person to person. In one case of a patient, as shown in FIG. 14, the optic papilla may be depressed at an angle with respect to the surface of the retina. In such a case, if, as in existing OCTs, a signal light beam is emitted in a direction perpendicular to the surface of the retina, the signal light beam is blocked by the entrance portion of the depressed part and, therefore, a sufficient amount of the light signal does not travel inside the depressed part. Even when the light signal travels inside the depressed part, a returning light beam of the reflected or scattered signal light beam attenuates and, therefore, an area where the intensity of the signal is significantly reduced (e.g., an area 1404) appears. In addition, even when the optic papilla is not at an angle, an area which the signal does not reach appears if the signal light beam is emitted at an angle different from the direction of the depressed part.
In addition, even when the signal light beam is perpendicularly emitted, the light beam is attenuated due to red blood cells if an image artifact caused by veins or bleeding appears. Thus, the intensity of a light beam reflected by a tissue under the blood vessels is significantly reduced.
If, as described above, the signal cannot be received, the layer structure cannot be identified. As a result, information required for diagnosis cannot be acquired.
PTL 1 describes a technology for capturing an image while changing the irradiation position through preliminary image capturing. However, since, in this technology, the image of an area in which the intensity of a signal light beam is low, as an area of a cataract, is not captured, the technology cannot be applied when the image of a target tissue is captured.